The removal of carbon dioxide from mixed gas streams is of great industrial importance and commercial value. Carbon dioxide is a ubiquitous and inescapable by-product of the combustion of hydrocarbons and there is growing concern over its accumulation in the atmosphere and its potential role in global climate change. While existing methods of CO2 capture have been satisfactory for the scale in which they have so far been used, future uses on the far larger scale required for significant reductions in atmospheric CO2 emissions from major stationary combustion sources, such as power stations fired by fossil fuels, makes it necessary to improve the energy efficiency of the processes used for the removal of CO2 from gas mixtures and thereby lower the cost of CO2 capture. According to data developed by the Intergovernmental Panel on Climate Change, power generation produces approximately 78% of stationary source emissions of CO2 with other industries such as cement production (7%), refineries (6%), iron and steel manufacture (5%), petrochemicals (3%), oil and gas processing (0.4%), and the biomass industry (bioethanol and bioenergy) (1%) making up the bulk of the total, illustrating the very large differences in scale between power generation on the one hand and all other uses on the other. To this must be added the individual problem of the sheer volumes of gas which will need to be treated. Flue gases generally consist mainly of nitrogen from combustion air, with the CO2, nitrogen oxides, and other emissions such as sulfur oxides making up relatively smaller proportions of the gases which require treatment. Typically, the wet flue gases from fossil fuel power stations can contain about 7-15 vol % of CO2, depending on the fuel, with natural gas giving the lowest amounts and hard coals the highest.
A variety of current efforts are underway to develop efficient, low cost technology for the capture and sequestration of carbon dioxide and other acid gases from dilute, low pressure gas streams. Conventionally, the separation of CO2 from dilute gas streams is done commercially by aqueous amine scrubbing systems. This technology is challenged for large scale implementation due to the high energy requirements of heating and cooling large volumes of water, and also by the corrosive salts that form as a product of the amine/CO2 chemistry. The latter factor coupled with solubility limits, creates a limit in CO2 capture efficiency of about 2 moles CO2/kg sorbent. Solid amine sorbents have greatly reduced energy requirements, little to no corrosivity problems, and uptake capacities in excess of 2 moles CO2/kg sorbent.
The literature on solid amine sorbents for CO2 capture is largely divided into supported amines based on the grafting or impregnation of various amine types either on the surface or in the pore volume of solid supports, or the polymerization of amine-containing monomers into amine-rich polymers or copolymers. The most likely process options for employing these solid sorbents in gas separation include pressure-swing adsorption, temperature-swing adsorption, or a combination of the two. In all of these cases, the rate of gas diffusion in and out of the sorbent particles is a critical factor in determining process economics. Slow diffusion rates translate into longer sorption/desorption cycle times and larger sorbent bed sizes. Both of these factors increase processing cost for a given gas feed rate.
Whereas the above solid amine sorbents have achieved acceptably high levels of CO2 sorption capacity, there is a general recognition that insufficient gas diffusion rates in these materials is a major limitation to their commercial utilization. Hence, a strong need exists for a sorbent design that imparts increased gas diffusion rates while maintaining high CO2 sorption capacity and cycle stability and at low cost.
U.S. Pat. No. 4,857,582 describes a process for preparing colloidal suspensions of polysiloxanes. The process includes introducing alkoxysilanes into an aqueous solution, along with an emulsifier, to form polysiloxanes. The alkoxysilanes can optionally include an aminoalkyl group, such as an N-(2-aminoethyl)-3-aminopropyl group. The process does not involve introduction of CO2.
A paper presented by Sae-Ung et al. as part of the 8th World Congress of Chemical Engineering described formation of a mesoporous material based on co-condensation of tetraethoxysilane and N-(2-aminoethyl)-3-aminopropyl triethoxysilane. Pores were formed in the mesoporous material by using a surfactant template as part of the condensation reaction and then extracting the template. During formation of the mesoporous material, CO2 was not introduced into the reaction mixture.
An article by Alauzun et al. that can be found at Chem. Mater., Vol. 20, pg 503 (2008) describes formation of amine-functionalized organosilicas. CO2 is introduced into the reaction mixture prior to triggering a condensation reaction. Due to carbamates formed from interaction of the CO2 with the amine-functionalized organosilicas, the reaction results in formation of a condensation product with at least some long range order.